WASHINGTON -- Fitch Ratings said Tuesday that it would keep its rating on long-term U.S. debt at the highest grade, AAA, and said the outlook remained stable. But it warned that it could lower the outlook to negative if Congress fails to trim future deficits.

Fitch's rating was the best given by the three major credit rating agencies. Last week, Standard & Poor's set off a maelstrom in the stock market after it downgraded long-term U.S. debt to the second-highest level, AA-plus, for the first time. The third agency, Moody's Investors Service, still lists the U.S. debt at AAA but says its outlook is negative.

The Obama administration welcomed Fitch's announcement. Presidential spokesman Jay Carney told reporters on Air Force One that Fitch's rating was in line with the administration's view.

Last week, S&P pointed to "political brinkmanship" in Congress over whether to raise the nation's debt ceiling as a reason for its downgrade. Early this month, with just hours to spare, President Barack Obama signed emergency debt-limit legislation to avoid a possible national default.

Fitch noted the rising proportion of U.S. debt relative to the economy. But it said it decided to maintain the AAA rating because the "key pillars" of U.S. creditworthiness remain intact, including a "flexible, diversified and wealthy economy."

Fitch estimated that the level of federal debt that's publicly traded -- not held in government trust funds -- will stabilize at around 85 percent of the economy by the end of the decade. That's higher than in other countries it rates as AAA and "at the limit" of what Fitch would consider consistent with a AAA rating, it said.

The proportion of debt that's publicly traded now stands at 72 percent of the economy.

After Fitch's announcement Tuesday, investors bought up Treasurys, driving down the yield on the 10-year Treasury note to 2.26 percent from 2.31 percent late Monday. Analysts suggested that the move reflected concerns about Europe's economies rather than relief at Fitch's announcement.

Treasury investors, in fact, may largely be ignoring the agencies' ratings of long-term U.S. debt. Investors appear more focused on the weakness of the U.S. economy, the likelihood of continued low inflation and greater confidence in U.S. Treasurys than in other countries' debt. After S&P's downgrade, for example, the U.S. bond market rallied.

"The credit agencies don't know any more about government budgets than the guy in the street who is reading the newspaper," said David Wyss, a former chief economist at S&P.

The government's debt rating, which measures the possibility that the United States will default, has become a political issue. Republican presidential candidates have cited S&P's downgrade as a symbol of what they call Obama's mishandling of the economy.

The downgrade came days after Republican lawmakers and the Obama administration agreed to raise the $14.3 trillion borrowing limit by more than $2 trillion in exchange for an equal amount of cuts to deficits over the next decade.

The first $917 billion in cuts were negotiated under the deal. But the rest would be handled by a 12-member committee of lawmakers appointed by Democratic and Republican leaders in the House and Senate. If the committee can't agree on cuts or Congress fails to approve them, automatic cuts of $1.2 trillion would go into effect starting in 2013.

The committee is supposed to come up with a plan by Thanksgiving. Congress is scheduled to vote on the measure before Christmas.

Fitch said it would revisit its rating after the committee makes its recommendations. A failure to agree on at least $1.2 trillion in deficit cuts would likely prompt Fitch to revise its outlook from stable to negative, the agency said.

A negative rating, Fitch said, indicates a greater than 50 percent chance of a downgrade within two years.

"Markets are going to remain worried about whether this committee can come together to make the hard decisions," said Christopher Rupkey, chief financial economist at Bank of Tokyo-Mitsubishi in New York.